Evidence of retrograde Mg-carpholite in the Phyllite-Quartzite nappe of Peloponnese from thermobarometric modelisation - Geodynamic implications

Abstract

A multiequilibrium thermobarometric approach was used to estimate the P-T conditions of successive metamorphic stages in carpholite-bearing blueschists that occur at the base of the Phyllite - Quartzite nappe of Peloponnese, in contact with the Ionian Plattenkalk. The calculated P-T conditions for successive metamorphic stages correlate well in samples from two different areas. A retrograde P-T path can be calculated, evolving from -e114 kbar, 400-450°C for chloritoid-bearing (carpholite-free) parageneses, to 11-13 kbar, 320-380°C for chloritoid + Mg-carpholite parageneses, ∼7.5 kbar, 260-275°C for Mg-carpholite + sudoite parageneses, and ∼3-4 kbar, ∼250°C for parageneses involving only phyllosilicates. Thermobarometric calculations suggest that Mg-carpholite is a retrograde phase in the Peloponnese. This feature has never been observed elsewhere in high pressure-low temperature carpholite-bearing schists. The calculated P-T paths indicate that the Mg-carpholite blueschists were subducted to ∼40km depth and maintained in a cool thermal gradient of ∼7°C/km during exhumation. © 2006 Lavoisier SAS. All rights reserved.